1. Field of the Invention
This invention relates to a signal processing method in a bar code reader, and a signal processing device for practicing the method.
2. Prior Art
In a bar code reader, a laser beam is applied to a bar code label, and scattering light beams reflected therefrom are converted into electrical signals by a light receiving unit, and the electrical signals are processed by a microcomputer to decode bar codes.
Bar codes employ thick bars of 1 mm in width, and thin bars of 0.2 mm in width. A bar code reader capable of reading both bar codes consisting of thick bars (hereinafter referred to as "thick bar codes") and bar codes consisting of thin bars (hereinafter referred to as "thin bar codes") has not yet been realized. Therefore, it is necessary to provide a bar code reader capable of reading thick bar codes, and a bar code reader capable of reading thin bar codes. More specifically, in the case where articles having bar code labels of thin bars, and articles having bar code labels of thick bars are mixed when conveyed, it is necessary to provide the above-described two separate kinds of bar code readers. The result of using both readers increases the equipment cost and the installation space is increased as much.
The reason why both of the thick bar codes and thin bar codes cannot be read with one kind of bar code reader is as follows:
The thick bar code reader uses a laser beam large in diameter. If the laser beam is applied to thin bar codes, then it will cover more than one thin bar. As a result, the output analog electrical signal of the light receiving unit is considerably low in sensitivity, and accordingly it is impossible to read the bar code.
On the other hand, in the case where a thin bar code reader is used to read thick bar codes, the above-described difficulty will not occur. However, since the laser beam diameter is smaller than the bar width, the sensitivity is so high that noise is picked up. More specifically, if a white bar of the bar code label is stained with ink or the like, then it is read as a black bar because the laser beam diameter is small. In addition, if a black bar of the bar code label is not well printed, then it will read as a white bar, so that the detection signal is outputted in the same manner as in the case of reading a white bar. In this case, the analog electrical signal outputted by the light receiving unit is made so irregular that it is difficult to read the bar code correctly.
On the other hand, a stationary type bar code reader is usually installed, for instance, on an assembling line in a factory, or a distribution line for transportation. The bar code reader operates to read the bar codes of labels bonded to articles being moved on by a conveyer.
However, the conventional bar code reader described above suffers from the further following difficulty: In order to allow the bar code reader to read bar codes even when the line speed is high, a method of bonding bar code labels to articles, and the size of them are limited.
More specifically, a bar code label must be bonded to an article in such a manner that the bar code is in parallel with the direction of scan of the laser beam with a tolerance of .+-.5.degree.. If the angle between the bar code and the direction of scan is large, then an effective reading region in the direction of the bars of a bar code (where all the bars can be scanned with the laser beam) is decreased. Even when the first part of the bar code thus inclined is scanned, the data read (which is ineffective because all the bars are not scanned) is processed; that is, it takes time to process the ineffective data (decoding loss time). Therefore, when it attempts to read the bar code again, often the bar code has passed away, with the result that the bar code cannot be read. Thus, it is essential that the inclination of the label is within the tolerance.
A bar code is provided with numerals and characters defined by JIS X9001 (which can be read directly by a person). In the case where an article with a bar code label is moved on the line in such a manner that the numerals and characters are ahead of the bar code in the direction of movement of the line, the data is processed with the numerals and characters read, and therefore similarly as in the above-described case, a decoding loss time occurs. Because of the decoding loss time, it is impossible to scan the bar code again. In addition, the bar code region which is effective in scanning the bar code is decreased, so that the read percentage is decreased. In order to eliminate the difficulty, the bar code label should be bonded to the article so that the numerals and characters are located behind the bar code in the direction of movement.
A method of eliminating the above-described difficulty is to increase the length of a bar code in the direction of movement of the label. However, the method is disadvantageous in that the label is increased in manufacturing cost as much, and depending on articles, it is impossible to stick such a long label on them.